1. Field of the Invention
This invention relates to an electric focusing apparatus for detecting the defocus amount of an object image formed by a photo-taking lens, calculating on the basis of the result of the detection the drive amount of a drive motor for driving the focusing optical system of the photo-taking lens (an optical system moved for focus adjustment), and controlling the drive motor on the basis of the result of the calculation to thereby effect the focus adjustment of the photo-taking lens or effect power focusing.
2. Related Background Art
An electric focusing apparatus provided in a photographing apparatus has an AF sensor for detecting a defocus amount or the like (or measuring a distance), a CPU for calculating and processing the detected information, a motor driven by the control of the CPU, a detecting device for detecting the drive amount of the motor and feeding back it to the CPU, body side and lens side gear trains for decelerating the driving of the motor, and a connecting member between these body side and lens side gear trains.
In such an electric focusing apparatus, there is always backlash in the gear train for decelerating the driving of the motor. Further, between the body side connecting member and the lens side connecting member, there is so-called backlash resulting from loosening or play. As a result, when the motor is driven in one direction and thereafter in the opposite direction, a state in which the motor is driven but the focusing lens is not moved exists in conformity with the above-mentioned backlash (including the so-called backlash). Therefore, the focusing lens is moved by only an amount smaller by the backlash than the amount of movement necessary for focusing (which amount will hereinafter referred to as the backlash amount). For the lens to be focused, the in-focus state must further be detected and the motor must be driven and thus, a long time is correspondingly required for the focusing operation. An apparatus for solving this problem is proposed in U.S. Pat. No. 4,537,487, and this apparatus is such that the data of the backlash amount of the aforementioned gear trains or the like is prepared in advance and correction is exerted on the drive amount of the motor during the focusing operation.
In the apparatus of the above-mentioned U.S. patent, however, the amount of backlash caused by the gear trains and the connecting members is the design value. That is, in an actual product, the backlash amount deviates from the design value due to the irregularity of manufacture, the change in parts with time, etc. and therefore, the apparatus proposed in the above-mentioned U.S. Patent cannot solve the problem completely.